Railroads have had several severe collisions and other accidents, some resulting in fatalities, when adequate spacing has not been maintained between rail vehicles. A major issue causing rail collisions involves vehicle operators failing to respond to a control point signal, such as a stop signal. For example, vehicle operators may fail to notice a control point signal due to visibility issues, equipment errors, operator maladies, or negligence. In such a situation, rail vehicles may continue to travel along a track not safe for travel, potentially resulting in disastrous collisions and/or accidents.
The Federal Railroad Administration established a system of rail regulations called positive train control (“PTC”). PTC is a system of functional regulations for monitoring and controlling train movements to provide increased safety. PTC, however, is an expensive system that is difficult to implement, particularly in light of the technology presently available. For public transit systems, PTC will generally increase travel time, passenger wait time, and decreased throughput.
Moreover, problems can also occur on railroads when sufficient spacing between rail vehicles is not properly maintained. Rail vehicle spacing can be monitored and maintained using various equipment and sensors installed on rail vehicles. For example, spacing between vehicles may be monitored by use of a global positioning system (“GPS”) that tracks and follows the location of rail vehicles on a rail track.
However, present methods and systems for controlling and enforcing rail vehicle separation can be insufficient, particularly under certain circumstances. For example, in subterranean tunnels, or subways, rail vehicles may not have a clear view of the sky, and therefore, may not be traceable by GPS methods. It can therefore be difficult for the operator of such a rail vehicle, to know the location, speed, and position of the rail vehicle relative to other vehicles on the train track with the accuracy necessary to operate safely and efficiently. Moreover, it can be difficult for a rail vehicle control system to properly enforce separation among rail vehicles on a railroad track if the precise location of each rail vehicle is not accurately known. For example, PTC can “dead-reckon” using the odometer in non-GPS areas, but there may be degradation in accuracy. This degradation may be acute in commuter rail applications where train spacing is less than with freight rail.